A solar battery is greatly expected to be a clean regenerative energy source, and researches have been conducted for practical application of a monocrystalline-silicon, polycrystalline-silicon or amorphous-silicon solar battery and a solar battery containing a compound such as cadmium telluride and indium-copper selenide. For prevalence of it as a household power source, however, any of these batteries faces many problems to be overcome, including a higher production cost, difficulty in ensuring raw material preparation, difficulty in recycling and difficulty in realizing a larger area. Therefore, there have been proposed solar batteries using an organic material in an attempt to achieve a larger area and a lower cost. However, any of these has a conversion efficiency of about 1%, which falls very short of practical use.
Under such circumstances, Graetzel et al. disclosed a photoelectric conversion element comprising semiconductor particles sensitized by a dye and a solar battery, as well as materials and technique for producing such a solar battery in Nature in 1991 (for example, Nature, Vol. 353, p. 737, 1991 (Non-patent document 1) and Japanese Laid-open Patent Publication No. 1989-220380 (Patent document 1)). The battery is a wet solar battery having a porous titania film sensitized by a ruthenium dye as a working electrode. This solar battery has the advantages that it can be provided as an inexpensive photoelectric conversion element because inexpensive materials can be used without highly purification, and that solar light can be converted into electricity over a wide visible light wavelength range because a dye having broad absorption is used. However, the conversion efficiency must be further improved for practical use, and therefore, it has been desired to develop a dye having a higher absorbance index and absorbing higher wavelength light.
Japanese Laid-open Patent Publication No. 2003-261536 (Patent document 2) by the present applicant has disclosed a mononuclear metal complex containing a dipyridyl ligand, which is a metal complex dye useful as a photoelectric conversion element.
“Current Technology in Dye-sensitized Solar Battery” (CMC Co., LTD., published on May 25, 2001, p. 117) (Non-patent document 2) has disclosed a polynuclear β-diketonate complex dye.
Japanese Laid-open Patent Publication No. 2004-359677 (Patent document 3), which is published on Dec. 24, 2004, has disclosed a polynuclear complex comprising a plurality of metals and a plurality of ligands wherein a bridge ligand (BL) coordinating to the plurality of metals has a coordination structure having a conjugated heterocyclic ring and a coordination structure without a conjugated heterocyclic ring, as a novel polynuclear complex improved in photoelectric conversion function of emitting electrons while receiving energy from active ray such as light.
There has been needed a useful and novel metal complex dye as a photoelectric conversion element.